* src/mkheader.c (write_special): Support @SOCKET_t@ substitution.
* src/gpgrt-int.h (gpgrt_stream_backend_kind_t): Add BACKEND_SOCK.
* src/gpg-error.h.in (struct _gpgrt_syshd): Use @SOCKET_t@.
* src/estream.c [HAVE_W32_SYSTEM] (estream_cookie_sock_t): New.
[HAVE_W32_SYSTEM] (func_sock_create, func_sock_read): New.
[HAVE_W32_SYSTEM] (func_sock_write, func_sock_seek): New.
[HAVE_W32_SYSTEM] (func_sock_destroy, func_sock_ioctl): New.
[HAVE_W32_SYSTEM] (estream_functions_sock, do_sockopen): New.
[HAVE_W32_SYSTEM] (do_sysopen): Call do_sockopen.